1. Field of the Invention
The invention relates to a liquid crystal display (LCD) device, and more particularly, to an LCD device including a light emitting diode (LED) as a light source.
2. Discussion of the Related Art
The liquid crystal display (LCD) devices are widely used for notebook computers, monitors, TV, and so on, because of their high contrast ratio and characteristics adequate to display moving images. The LCD devices use optical anisotropy and polarization properties of liquid crystal molecules to display images.
The LCD devices require a liquid crystal panel including first and second substrates and a liquid crystal layer interposed therebetween. An arrangement of the liquid crystal molecules in the liquid crystal layer is changed by an electric field induced in the liquid crystal panel to control light transmissivity.
Generally, an additional light source is required because the liquid crystal panel is a non-emissive-type display device. Accordingly, a backlight unit is disposed under the liquid crystal panel. The LCD device displays images using light produced by the backlight unit and supplied to the liquid crystal panel. A cold cathode fluorescent lamp (CCFL), an external electrode fluorescent lamp and a light emitting diode (LED) are used as a light source of the backlight unit. Among these light sources, the LEDs are widely used because of advantages in a size, power consumption and reliability.
FIG. 1 a cross-sectional view of the related art LCD device using an LED as a light source. Referring to FIG. 1, the LCD device includes a liquid crystal panel 10, a backlight unit 20, a main frame 30, a bottom frame 50 and a top frame 40. The liquid crystal panel 10 includes first and second substrates 12 and 14 facing each other and a liquid crystal layer (not shown) therebetween. First and second polarizing plates 19a and 19b for controlling a polarization direction of light, are respectively disposed at front and rear sides of the liquid crystal panel 10.
The backlight unit 20 is disposed at a rear side of the liquid crystal panel 10. The backlight unit 20 includes an LED assembly 29 arranged along a length direction of at least one edge of the main frame 30, a reflective sheet 25 of white or silver color on the bottom frame 50, a light guide plate 23 on the reflective sheet 25 and an optical sheet 21 on the light guide plate 23. The LED assembly 29 is positioned at one side of the light guide plate 23 and includes at least one LED 29a emitting white light and an LED printed circuit board (PCB) 29b where the LED 29a is disposed. The optical sheet 21 includes a plurality of sheets.
The liquid crystal panel 10 and the backlight unit 20 are combined using the main frame 30 that can prevent movement of the liquid crystal panel 10 and the backlight unit 20. The top frame 40 cover edges of the liquid crystal panel 10 and sides of the main frame 30, so the top frame 40 can support and protect of the edges of the liquid crystal panel 10 and sides of the main frame 30. The bottom frame 50 covers back edges of the main frame 30, so the bottom frame 50 is combined with the main frame 30 and the top frame 40 for modulation.
FIG. 2 is an enlarged cross-sectional view of an “A” portion in FIG. 1. Referring to FIG. 2 with FIG. 1, the LED 29a is positioned at one side of the light guide plate 23 and arranged on the PCB 29b. The LED 29a has a fixed position and faces a side portion of the light guide plate 23 such that light emitted from the LED 29a is projected to the light guide plate 23. To provide a space for the LED assembly 29, the bottom frame 50 has a bending portion. Namely, the bottom frame 50 is bent upwardly. The LED assembly 29 is fixed in the bending portion of the bottom frame 50 using an element, for example, a double coated tape. The above backlight unit may be called as a side top-view type.
The light emitted from the LED 29a is projected to the light guide plate 23 and refracted or reflected to be projected onto the liquid crystal panel 10. The light is processed into an uniform plane light source during passing the optical sheet 21.
On the other hand, a temperature of the LED 29a is rapidly increasing with an operating time. The temperature increasing of the LED 29a causes problems in a lifetime and brightness uniformity of the LED 29a. Accordingly, when using the LED 29a as a light source, it is very important to provide a heat-radiating member for the LED 29a. 
However, since there is no heat-radiating member for the LED in the conventional LCD device, a temperature increasing problem in the LED is generated with an operation. Accordingly, there are disadvantages in a lifetime, brightness uniformity and displaying image quality in the LCD device using the LED.
To resolve these problems, a metal core printed circuit board (MCPCB) having a heat-radiating property is used for the LED PCB 29b, and the bottom frame 50 is formed of a high thermal conductive material, for example, aluminum (Al). Unfortunately, there is a disadvantage in production costs.